1. Field of Invention
The present invention relates to a universal serial bus (USB) charging circuit and a charging method, in particular to a USB charging circuit having a simpler circuit structure and providing better protection to other peripheral devices connecting with the USB host, and a charging method.
2. Description of Related Art
In current state of the art, a USB does not only provide signal transmission but is also capable of providing power, e.g., to charge a mobile phone. However, because a USB host is usually dynamically connected with multiple different devices, its load varies dynamically; it is therefore preferred that a USB host provides a charging current which changes according to the load, instead of providing a constant current.
US patent publication No. 2006/0033474 discloses a USB charging circuit which provides a charging current that changes according to the load. A simplified form of this conventional circuit is shown in FIG. 1, in which the charging circuit 1 includes a transistor 11 operating as a charging switch, to charge a battery 10 from a voltage node Vbus at the USB host. A loop control circuit 12 controls the transistor 11 to determine its conduction, i.e., the current charging the battery 10. The loop control circuit 12 determines the charging current according to a current sense signal obtained from the path A, a voltage signal obtained by dividing the voltage at the node Vbus, and an upper limit set by a current setting circuit 14; the upper limit may be decided by an external input. The loop control circuit 12 also obtains the voltage of the battery 10 from the node B, and decides whether to switch the charging mode depending on whether the voltage at the node B reaches a predetermined level. An error amplifier 16 compares the dividend voltage from the node Vbus with a reference voltage Vref, to determine the difference between the voltage at the node Vbus and a preset minimum voltage.
The conventional circuit is characterized by a compensation circuit 18, and a charge reduction loop 20 formed among the location for sensing current, the resistor R1, the parallel circuit of the compensation circuit 18 and the error amplifier 16, and the summation circuit 19. When the charging current is too high such that the voltage at the node Vbus drops below the preset minimum voltage, the charge reduction loop 20 functions to bring the voltage at the node Vbus back to or above the preset minimum voltage. In other words, this conventional circuit provides current as largest as possible to charge the battery 10 from the node Vbus, while maintaining the voltage at the node Vbus above the preset minimum voltage.
Although this conventional circuit might provide a charging current as largest as possible, it has the following drawbacks: first, it has a complicated circuit structure because it needs to constantly detect the voltage variation at the node Vbus; second, at an instance when the charging current is too high that the voltage at the node Vbus drops below the preset minimum voltage, a peripheral device which is connected with the USB host may be adversely affected, suffering data loss if data is being transmitted from/to the device.
In view of the foregoing drawbacks in the prior art, it is desired to provide a USB charging circuit having a simpler circuit structure and providing better protection to other peripheral devices connecting with the USB host, and a charging method.